In servo-controlled injectors for internal combustion engines and, in particular, common-rail injection systems, injection control is performed by the aid of a solenoid valve. The solenoid valve controls the outflow of fuel from the control chamber of an injection nozzle. A servo-controlled injector according to the prior art is illustrated in FIG. 1.
FIG. 1 depicts the schematic structure of a modular common-rail injection system. Fuel is sucked from the fuel tank 1 by a prefeed pump 2 and, by a high-pressure pump 3, is brought to the required system pressure and fed to the injector 4. The injector 4 is comprised of an injection nozzle 5, a throttle plate 6, a solenoid valve 7, an injector body 8 equipped with a high-pressure accumulator (not illustrated), and a nozzle clamping nut 9 for holding the components together. In the idle position, the solenoid valve 7 is closed such that high-pressure fuel flows from a high-pressure bore 10 via a transverse groove 11 and an inflow throttle 12 into the control chamber 13 of the nozzle 5, yet while blocking the outflow from the control chamber 13 via the outflow throttle 14 on the valve seat 15 of the solenoid valve 7. The system pressure applied in the control chamber 13 along with the force of the nozzle spring 16 presses the nozzle needle 17 into the nozzle needle seat 18, thus closing the injection holes 24.
As the solenoid valve 7 is actuated by activating the electromagnet 25 and the solenoid valve member 27 is lifted from the solenoid valve seat 15 against the force of a solenoid valve spring 26, it clears the passage through the solenoid valve seat 15, and fuel is flowing from the control chamber 13 back into the fuel tank 1 through the outflow throttle 14, the armature chamber 19 of the solenoid valve, the outflow gap 20, the relief bore 21 and the low-pressure bore 22. Within the control chamber 13, an equilibrium pressure defined by the flow cross sections of the inflow throttle 12 and the outflow throttle 14 adjusts, which is so low that the system pressure applied in the nozzle chamber 23 is able to open the nozzle needle 17, which is guided within the nozzle body 29 in a longitudinally displaceable manner, thus clearing the injection holes 24 and effecting injection.
Due to the geometry of the solenoid valve 7, the magnetic coil 28 proper (which consists of a plastic winding support and the copper wire windings) is in direct contact with the fuel such that damage may be caused due to cavitation erosion at the occurrence of cavitation in the system and, in particular, in plastic components as are used for the winding support of the magnetic coil 28. Cavitation develops in the following manner:
When the valve seat 15 is open, the armature 27 directly abuts on the stroke stop of the magnet pot 25 such that only a very small residual air gap of 50-80 μm will remain between the armature 27 and the magnet pot 25. When the magnet is deactivated, the force of the valve spring 26 causes such a strong negative pressure in said residual air gap that the fuel present in the residual air gap will at least partially evaporate over a short time. After this, the pressure will again rise, thus causing the vapour bubbles to explode. If this happens on the surface of the winding support of the magnetic coil 28, cavitation erosion will occur there, which with long running times may reach as far as to the copper windings of the magnetic coil 28 and finally destroy the same, thus causing a failure of the solenoid valve 7.
The present invention now aims to avoid such damage due to cavitation erosion.